By definition a monolithic microwave integrated circuit (“MMIC”) device contains all of the active and passive circuit elements and associated interconnections formed either in site on or within a semi-insulating substrate, such as a semiconductor, or insulating substrate by one or more well known deposition processes. Typically the MMIC device is formed on a semiconductor chip (or group of semiconductor chips) and the chip is (or chips are) assembled into a package.
A first step in the packaging of a semiconductor chip for a MMIC circuit application is to attach the chip to a metal carrier, suitably a carrier of conventional aluminum alloys, which serves as the floor of the chip package. Typically, the chip in that application is formed of either Gallium Arsenide (“GaAs”) or Indium Phosphide (“InP”) with a thin metal layer, referred to as the back metal layer, covering the back side of the chip. Direct attachment of the chip to the carrier by means of solder or epoxy is impractical due to the thermo-mechanical mismatch between the GaAs or InP material of the chip and the aluminum alloy carrier. That mismatch results from the large difference in the respective coefficients of thermal expansion (“CTE”) of both GaAs and InP with the CTE of aluminum. Since the backmetal layer of the chip is so thin relative to the thickness of the semiconductor material, the CTE of the thicker semiconductor material predominates, and the CTE of the backmetal layer may be ignored. Because of the foregoing difference in the CTE, upon heating, the aluminum alloy carrier expands faster than the semiconductor chip. That in turn produces undue levels of stress on the chip when the chip and carrier are rigidly bonded together by a bonding agent that is incapable of providing stress relief. In that circumstance, the chip usually cracks, rendering the electronic circuits fabricated on the chip inoperative.
One known means of bonding metal to metal as could possibly be used to attach chip-to-carrier is solder; another is with conductive epoxy. Solder, however, possesses a relatively high elastic modulus and low percent elongation characteristic, and, therefore, is incapable of acting as a stress buffer between the chip and the aluminum carrier during thermal expansion. The same characteristic and inability generally exists when epoxy is used as the bonding agent. Although epoxy has been found acceptable in some instances with some chips, the reliability as a thermal stress buffer in applicant's view has not been universally consistent.
For the foregoing reasons, either A40 material, an aluminum silicon alloy, or Kovar material, an iron-nickel-cobalt alloy, are currently used as carrier materials. Those known materials are found to decrease the CTE mismatch between chip and carrier. The A40 material is expensive and brittle. The brittleness makes the material difficult to machine to the geometry desired for the carrier. Outweighing the foregoing disadvantage, the CTE of the A40 material is a better match to that of the GaAs and InP chips, and makes that bonding material desirable for a chip carrier. Kovar has an excellent CTE match to that of GaAs and InP, but possesses a low thermal conductivity and high density. That material is also hard and, hence, is difficult to machine.
Accordingly, an object of the present invention is to minimize thermally induced stress on packaged MMIC chips.
Another object of the invention is to provide a wafer-level package for MMIC chips that minimizes or avoids the need for hermetically sealed packages.
The chip package contains the carrier layer, that serves as the package floor, but also side walls and a cap or lid. For space borne applications the package is often required to be hermetically sealed so as to prevent entry of gas or other unwanted particles. Hermetically sealed packages require the lid to be welded to the side walls and is otherwise quite rugged, resulting in a package that is relatively heavy, housing a chip that is relatively light in weight, a package that weighs considerably more than one that is not hermetically sealed.
An additional object of the invention is to protect the MMIC chip from moisture, gas or other unwanted particles without the need to hermetically seal the package.